The present invention relates to defibrillator circuits for generating trapezoidal waveform therapeutic electrical pulses. More specifically, the present invention relates to such a defibrillator circuit which is particularly useful in portable defibrillator devices used under emergency circumstances to apply external defibrillation pulses to heart attach victims suffering from ventricular fibrillation.
Two basic defibrillation pulse waveforms are the damped sinusoidal waveform and the trapezoidal waveform. To deliver equal amounts of therapeutic energy to a patient, the maximum voltage required for effective defibrillation using a trapezoidal defibrillation waveform is typically much less than the maximum voltage required when a damped sinusoidal defibrillation waveform is used. Consequently, a trapezoidal waveform defibrillator circuit has a number of advantages over a damped sinusoidal defibrillator circuit. In the first plate, in comparison to damped sinusoidal waveform defibrillators, lower voltages applied to circuit components of trapezoidal waveform defibrillators reduce the stress on parts subjected to these voltages and thereby reduces the likelihood of component failure. Also, because of these lower voltages, smaller, less costly and more commonly available components may be used in a defibrillator circuit employing this trapezoidal waveform technique. In addition, due to the lower voltages involved, it is easier to electrically isolate high voltage sections of a trapezoidal defibrillator circuit for operator safety. Moreover, trapezoidal waveform defibrillator circuits cah have lower internal impedances than damped sinusoidal defibrillator circuits. Therefore, energy is used more efficiently because a smaller proportion of the energy intended to be applied to a patient is dissipated internally by components of the defibrillator circuit.
One prior art electro-mechanical circuit for a trapezoidal waveform ventricular defibrillator is shown in U.S. Pat. No. 3,359,984 of Daniher et al. In this device, a spring biased armature is cocked to a first position. During defibrillation, the armature is released and slides into engagement with a first set of contacts. This couples a first charged capacitor through these contacts to a patient and commences the application of a trapezoidal waveform defibrillation pulse. As the armature continues to slide, the circuit through the first set of contacts is broken and a second set of contacts is engaged. When this occurs, a second charged capacitor is coupled through the second set of contacts to the patient and another defibrillation pulse is applied. This latter pulse continues until such time as the armature slides out of engagement with the second set of contacts. It is difficult to control the amount of energy applied to a patient by such an apparatus. In addition, the repeated making and breaking of high voltage contacts can result in arcing which wears the armature and contacts of such a device.
Another prior art defibrillator of interest is disclosed in U.S. Pat. No. 3,886,950 of Ukkestead et al. In Ukkestead, a plurality of capacitors are connected for charging in parallel and discharging in series upon commencement of a defibrillation discharge cycle. At the start of a defibrillation cycle, the charge from the capacitors is applied to a patient. When the desired energy has been delivered, a silicon controlled rectifier (SCR) based truncate circuit shunts the capacitors to discharge all capacitors that have not previously been discharged during the delivery of a defibrillation pulse.
In addition, U.S. Pat. No. 3,706,313 of Milani et al. discloses a defibrillation circuit in which a charged capacitor, at the start of the defibrillation pulse, is discharged through a patient and a first SCR. To terminate the defibrillation pulse, a second SCR is turned on to short circuit the first SCR and discharge any remaining charge on the capacitors through a path which does not include the patient.
With this approach, as well as with the Ukkestead approach, residual energy in the capacitors, which is typically twenty-five percent of the originally stored energy, is wasted. In many cases, it is desirable to apply more than one defibrillation pulse to a patient. To administer subsequent defibrillation pulses using the Milani and Ukkestead devices, it is necessary to recharge the capacitors of these devices from a totally discharged state. This requires additional time and current from batteries included in a portable defibrillator. This latter drawback can limit the number of defibrillation pulses available from the device before the batteries need recharging.
The background of the invention portion of the Milani disclosure mentions the possibility of turning off a first SCR after a portion of the charge on a capacitor has been discharged to stop a defibrillation pulse. Milani also recites that a substantially trapezoidal waveform defibrillation pulse may be produced in this manner. However, Milani states that such an approach is unreliable because the turn-off mechanisms of SCR devices are less reliable than the turn-on mechanisms. Milani thus teaches away from trying to turn off an SCR to terminate a defibrillation pulse.
Therefore, a need exists for an improved defibrillator circuit for applying trapezoidal waveform defibrillation pulses to a patient.